This paper contains a general review of the various methods which have been proposed for the estimation of basic slip factors for centrifugal impellers. As a result of this study, it is concluded that the classical method proposed by Busemann in 1929 is still the most generally applicable prediction for the basic slip factor of centrifugal impellers. The paper then presents a very simple empirical expression, which fits the Busemann results extremely well over the whole range of practical blade angles and number of blades up to a limiting inlet-to-outlet radius ratio for the impeller. An empirical correction factor is also proposed for conditions which exceed this limiting radius ratio. Tabular comparisons of slip factors, with test data (where available), are given for over 60 pump and compressor impellers which have been cited previously in the literature, and the author has added data for several more compressor stages from his own experience.